The present invention relates to a process for washing a cellulose pulp from alkali digestion.
The washing of a cellulose pulp from alkali digestion is usually performed with water or process condensates in a system consisting of countercurrently coupled washers. Part of the wash losses, i.e., that part of the inorganic and dissolved organic material which remains in the fibrous material after the wash remains chemically combined, absorbed or enclosed in the fibrous material so that it cannot be removed with water. By lowering the pH value of the washing solution by an acid addition that part which cannot be removed with water can be released. It has, however, been suggested that certain detrimental effects are thereby produced. Dissolved lignin or other organic substance precipitates when the pH value is lowered, and if the fibers have been treated by sulfate, polysulfide or hydrogen sulfide digestion, hydrogen sulfide gas is released under the effect of a reaction between the acid and the sulfides of the spent liquor. An acid wash is applied when the object is to separate trace elemets, such as heavy metals which have been noted to catalyze the decomposition of cellulose in so-called oxygen bleaching or delignification. This stage is placed separately between the wash and the subsequent oxygen gas treatment. The filtrate from this treatment is fed directly into the sewage system.
The general trend in the cellulose industry is to decrease liquid and gaseous wastes by closing liquid circulation systems in the process. This means that contamination increases in the circulation system, which can be compensated for on the liquid side by adopting several countercurrently coupled washing stages. This, however, considerably increases production costs.